For some time now, wideband analog active filters have been synthesized by placing integrators in the forward filter path and modifying coefficients of the denominator polynomial, using negative feedback circuitry which is summed at the input of the filter. To synthesize a multi-pole filter network, first and/or second-order modular block filter networks have been cascaded to obtain the desired overall filter order.
As illustrated in FIG. 1a, it is well known to construct second-order filters 100 by cascading two integrators 110 and 112 with each output fedback to the originating input via separate paths 114 and 116, each with its own scale factor. Each of the integrator circuits 110 and 112 typically consists of a Gilbert cell with a capacitor output load.
The gain (set at Iin) of each integrator is proportional to the transconductance characteristics of the Gilbert cell, i.e., gain equals gm/sC. The gm of the Gilbert cell is proportional to its tail current, which enables the resonant frequency to be varied proportionally. By feeding the nodes signals forward, summed with the proper scale factor, zeros are constructed.
It is also well known to construct a first-order filter circuit as illustrated in FIG. 1b. In this circuit, a single integrator 122 is used with its output fedback to the originating input via path 124, with a scale factor K1.
One difficulty in these designs is that, due to the long feedback paths, accumulation of stage delays and parasitics, the ability to synthesize high frequency, Hi-Q filter networks is limited.
In wideband signal filter circuits, where the signal spectrum exceeds 20 MHz, 10 Ghz or higher bipolar processes are required. Currently, there are commercially available chips (7 poles with 2 zeros) which implement Bessel type, LO-Q filters, that operate up to 13 MHz. However, each of these commercially available chips requires an excessive amount of bipolar circuitry which is costly in terms of physical chip space, manufacturing expense and reliability.
The present invention is directed to overcoming these above-referenced deficiencies in known tunable wideband signal filter circuits.